Deciphering records of geomagnetic reversals Jean-Pierre Valet, Alexandre Fournier To cite this version: Jean-Pierre Valet, Alexandre Fournier. Deciphering records of geomagnetic reversals. Reviews of Geophysics, American Geophysical Union, 2016, 10.1002/2015RG000506. insu-02400962 HAL Id: insu-02400962 https://hal-insu.archives-ouvertes.fr/insu-02400962 Submitted on 9 Dec 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. PUBLICATIONS Reviews of Geophysics REVIEW ARTICLE Deciphering records of geomagnetic reversals 10.1002/2015RG000506 Jean-Pierre Valet1 and Alexandre Fournier1 Key Points: 1Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, Paris, France • We examine and assess an updated database of geomagnetic reversal records fi ’ • We discuss robust and controversial Abstract Polarity reversals of the geomagnetic eld are a major feature of the Earth s dynamo. Questions features of the transitional field remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic • Future work should involve millimeter- field during a polarity transition. A large number of paleomagnetic reversal records have been acquired size specimens and physics-based fi models during the past 50 years in order to better constrain the structure and geometry of the transitional eld. In addition, over the past two decades, numerical dynamo simulations have also provided insights into the reversal mechanism. Yet despite the large paleomagnetic database, controversial interpretations of records of the transitional field persist; they result from two characteristics inherent to all reversals, both of which Correspondence to: J.-P. Valet, are detrimental to an ambiguous analysis. On the one hand, the reversal process is rapid and requires [email protected] adequate temporal resolution. On the other hand, weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records. This paper is aimed at reviewing critically the main Citation: reversal features derived from paleomagnetic records and at analyzing some of these features in light of Valet, J.-P., and A. Fournier (2016), numerical simulations. We discuss in detail the fidelity of the signal extracted from paleomagnetic records Deciphering records of geomagnetic and pay special attention to their resolution with respect to the timing and mechanisms involved in the reversals, Rev. Geophys., 54, 410–446, doi:10.1002/2015RG000506. magnetization process. Records from marine sediments dominate the database. They give rise to transitional field models that often lead to overinterpret the data. Consequently, we attempt to separate robust results Received 10 OCT 2015 (and their subsequent interpretations) from those that do not stand on a strong observational footing. Accepted 17 MAR 2016 Finally, we discuss new avenues that should favor progress to better characterize and understand transitional Accepted article online 24 MAR 2016 Published online 17 MAY 2016 field behavior. 1. Introduction Geomagnetic reversals are an extreme manifestation of the variability of the geomagnetic field created and sustained by dynamo action in Earth’score.Thisvariabilityspansavastrangeoftimescales,fromdays to millions of years. Composite power spectra of dipole fluctuations constructed from observations [Constable and Johnson, 2005] and numerical dynamo simulations [Olson et al., 2012] point to an intermedi- ate frequency band with periods from a few centuries to a few tens of kiloyears, where, according to Olson et al. [2012], the energy density increases with the period in an almost quadratic fashion; reversals (and excursions, i.e., failed attempts at reversal) lie within that transitional band (that of paleosecular variation). No obvious separation of excursional to reversing behavior exists, and the question remains whether reversals reflect core processes that are different from those at work behind regular paleosecular variation. In this review, our primary goal is to outline the constraints that current (and future) observations may help to place on these processes, by means of a detailed description of the so-called transitional field, i.e., the global geomagnetic field one would map out during a reversal. Good knowledge of transitional field beha- vior is also key to predicting how the Earth system (and, in particular, the magnetosphere) would evolve in the event of a reversal (see, e. g., the discussion by Constable and Korte [2006] and the review by Glassmeier and Vogt [2010]). The first records of geomagnetic reversals were published more than 50 years ago [Van Zijl et al., 1962; Momose, 1963]. Since then an increasing number of records have been obtained from rock types with diverse physicochemical processes of formation and magnetization. Most studies have dealt with sediments, no ©2016. The Authors. more than two dozens from lava flows, and three records from intrusions [Dodson et al., 1978]. In most cases, This is an open access article under the identification of reversals relies on detailed magnetostratigraphic profiles for sediments and on radiometric terms of the Creative Commons fl Attribution-NonCommercial-NoDerivs ages for lava ows. Considerable progress has been achieved over the past 20 years by tuning carbonate License, which permits use and distri- variation in sedimentary sequences to Earth’s orbital variations with a precision better than 20 kyr [Imbrie bution in any medium, provided the et al., 1992]. However, this time span exceeds the duration of reversals. It is, therefore, difficult to determine original work is properly cited, the use is fi non-commercial and no modifications rapid geomagnetic variations with adequate temporal precision. These differences are ampli ed by weak or adaptations are made. transitional fields that contribute to increase recording complexities. VALET AND FOURNIER GEOMAGNETIC REVERSALS 410 Reviews of Geophysics 10.1002/2015RG000506 Like any scientific topic, our knowledge about field reversals has been acquired through several steps. A first important objective was to determine whether the field would maintain a dipolar character when reversing. Early studies [Creer and Ispir, 1970] proposed that the field had retained dipolar configurations during seven polarity reversals recorded at widely separated sites. The motion of the pole through the Indian Ocean was simulated by a model of three dipoles that was rapidly discarded with accumulation of new results, mostly from sediments. The dipolar nature of the transition was also questioned by the first measurements of absolute paleointensity which showed that polarity changes are accompanied systema- ticallybylowfield values [van Zijl et al., 1962; Momose, 1963; Coe, 1967; Lawley, 1970; Larson et al., 1971; Niitsuma, 1971; Dagley and Wilson, 1971]. These observations were reinforced by the dominance of low magnetization intensities associated with transitional lava flows [Kristjansson and McDougall, 1982]. The first compilation of volcanic records led Dagley and Lawley [1974] to favor a nondipolar transitional field. By then a succession of models had been proposed and tested against an increasing number of paleomagnetic data. Knowledge of field geometry during polarity reversals remains limited due to the difficulty of acquiring well- documented records of the same transition from widely distributed locations, particularly from the Southern Hemisphere which remains relatively depleted of data. Despite advances in determining successive field directions, along with relative and absolute paleointensity changes during transitions, interpretations are hampered frequently by relatively poor knowledge of the processes involved in magnetization acquisition, especially for sediments, and by the absence of a detailed chronology for lava flow sequences. The last reversal (Matuyama-Brunhes, M-B) has been the focus of much attention, because this is the first major event that can be recognized in marine sediment cores. The successive positions of the virtual geo- magnetic pole (VGP)—the apparent location of the pole assuming an axial geocentric dipole field—that define the VGP path have been commonly used to infer features of the transitional field. The first five pub- lished M-B records showed some tendency of VGP paths to lie near the longitude of their sampling site. Hoffman and Fuller [1978] concluded that the transitional field was predominantly controlled by degree 2 or 3 zonal harmonics and, therefore, it was axisymmetric but not dipolar. As more detailed records of the last reversal and other events became available, these models could not be defended any more. It was then con- sidered that the field was neither axisymmetric [Hoffman, 1979; Fuller et al., 1979; Valet et al., 1988a; Prévot, 1977; Prévot et al., 1985; Clement and Kent, 1984] nor dipolar but likely controlled by nonzonal harmonics of low degree [Hoffman, 1984]. Unfortunately, in this case a large distribution of sites that record the same reversal is needed to determine the exact